Don't reexport bigint from util

ethcore/evm/src/interpreter/shared_cache.rs

@@ -17,7 +17,8 @@
 use std::sync::Arc;
 use hash::KECCAK_EMPTY;
 use heapsize::HeapSizeOf;
-use util::{H256, Mutex};
+use bigint::hash::H256;
+use util::Mutex;
 use util::cache::MemoryLruCache;
 use bit_set::BitSet;
 use super::super::instructions;